The invention relates to a ball screw mechanism comprising a screw shaft and a nut member.
A ball screw mechanism is known as a mechanism which converts rotational motion to linear motion. As an example of such a ball screw mechanism, the whole configuration of a circulation tube type ball screw mechanism will be described with reference to FIG. 1. FIG. 1 is an axial section view of a ball screw mechanism. In the figure, a screw shaft 1 which is partly shown is a shaft member which has in the outer periphery a spiral groove (thread groove) 1a having a section shape similar to a Gothic arch as described later. A nut 2 serving as a nut member is a cylindrical member which has in the inner periphery 2e a spiral groove (thread groove) 2a corresponding to the spiral groove (thread groove) 1a of the screw shaft 1, and a spiral ridge 2d defined between the adjacent spiral grooves 2a. Although not illustrated, two through holes elongate from the upper face 2c of the nut 2 to the spiral groove (thread groove) 2a. Ends of a ball circulation tube 4 having a U-like shape as indicated by a phantom line are fittingly inserted into the through holes, respectively.
The screw shaft 1 is passed through the inside of the inner periphery 2e of the nut 2 so that the spiral groove (thread groove) 1a opposes to the spiral groove (thread groove) 2a of the nut. A number of balls 3 are rotatably housed in a trackway defined by the two opposed thread grooves.
When the nut 2 and the screw shaft 1 perform relative spiral movement, the balls 3 repeat circulation in which the balls rotatingly move along the trackway formed between by the spiral grooves (thread grooves) 1a and 2a, are scooped up from the trackway while being guided by a tongue portion (not shown) formed at one end of the ball circulation tube 4, to be directed into the ball circulation tube 4, pass through the tube, and then return to the trackway via the other end of the nut.
In addition to this, FIG. 6 shows an example of an end cap type ball screw device of the prior art. In the conventional example, a screw shaft 1 having a spiral groove (thread groove) 1a in the outer peripheral surface is threadedly engaged with a cylindrical ball screw nut 10 having in the inner peripheral surface a spiral groove (thread groove) 2a opposing to the spiral groove (thread groove) 1a of the screw shaft, via balls 3 which rotatingly move in the mutually opposing spiral grooves (thread grooves) 1a and 2a. The ball screw nut 10 comprises two kinds of members, i.e., a nut member 402, and disk-like ball circulation members (so called end caps) 11 which are detachably joined to the end faces of the nut member 402. A ball return passage 12 which consists of a through hole elongating in the axial direction is disposed in a thick portion of the nut member 402. In each of the end faces of the ball circulation member 11 where the member is joined to the nut member 402, disposed is a curved path 13 through which the spiral grooves (thread grooves) 1a and 2a communicate with the ball return passage 12.
When the screw shaft 1 and the ball screw nut 10 are relatively rotated, the balls 3 rotatingly advance in the two opposing screw grooves 1a and 2a of the screw shaft 1 and the ball screw nut 10 so as to repeat circulation in which the balls pass through the curved paths 13 disposed in the ball circulation members at the ends, and the ball return passage 12 disposed in the nut member 402, to return to the original position.
Since the balls which rotate in accordance with the rotation of the screw shaft move along the trackway, continuation of the relative spiral movement of the nut (or the nut member) and the screw shaft causes the balls to be discharged from the nut in due course of time. When the nut (or the nut member) is to be moved by a considerably long distance, therefore, any ball screw mechanism must be provided with a circulation unit such as a ball circulation tube which returns balls discharged from one end of the nut (or the nut member) to the other end of the nut. However, the provision of such a circulation unit produces a problem peculiar to a ball screw mechanism.
Before the discussion of the problem, the relationship between balls and thread grooves is first described. FIG. 2 is an enlarged section view showing the vicinity of a thread groove of the ball screw mechanism of FIG. 1, along a direction perpendicular to the thread groove. Referring to the figure, a ball 3 is disposed between the thread groove 1a of the screw shaft 1 and the thread groove 2a of the nut 2.
As apparent from FIG. 2, the sections of the thread grooves 1a and 2a are not parts of a perfect circle, and have a shape which is a so-called Gothic arch and each of which is configured by combining two arcs (called flanks) with each other. Specifically, the sections of the thread grooves 1a and 2a constitute a shape in which arcs of a radius of curvature Rc are arranged in a laterally symmetrical manner. When the radius of the ball 3 is indicated by R, the relationship of Rc &gt;R is held.
In view of the above-mentioned relationship between the radius R of the ball 3 and the radius of curvature Rc, the thread grooves 1a and 2a and the ball 3 are contacted with each other at four points, that is the points N1, N2, S1, and S2 in the FIG. 2. According to this configuration, a controlled pre-load can be easily applied to the balls, so that a back lash can be eliminated. When the pre-load is applied, the reaction forces produced at the four points balance with each other.
Hereinafter, the problem peculiar to a ball screw mechanism will be described with reference to the drawings.
FIG. 3 is an enlarged view of an end portion of the nut 2 of the end cap type ball screw mechanism as seen in the axial direction, and FIG. 4 is a view of the nut 2 of FIG. 3 as seen in the direction of the arrow IV. As apparent from comparison of FIG. 2 with FIGS. 3 and 4, since the thread groove 2a has a lead angle .theta. and the thread groove is cut by the end face 2b of the nut 2 which is perpendicular to the axis, the thread groove 2a has an opening shape which elongates in the peripheral direction.
In FIGS. 3 and 4, the ball 3 which rotates in the thread groove 2a is indicated by phantom lines. The relative spiral movement of the nut and the screw shaft (not shown) causes the ball 3 to rotate so that the center of the ball 3 moves in the sequence of the positions C, B, and A, and the ball is finally discharged from the nut 2. The line N1.sub.TR indicates the locus of the contact point N1 between the ball 3 and the thread groove 2a, and the line N2.sub.TR indicates the locus of the contact point N2 between the ball 3 and the thread groove 2a.
Until the center of the ball 3 reaches the position C, the ball is contacted with the thread groove 2a of the nut 2 at the two points, and also with the thread groove of the screw shaft (not shown) at two points. In other words, the contact relationship between the ball and the thread grooves is in the normal state shown in FIG. 2.
At the timing when the center of the ball 3 passes the position C, however, the line N2.sub.TR is interrupted at the point N2-C as shown in FIG. 4. By contrast, the line NLTR further elongates to continue to the point N1-A.
FIG. 5 is a section view similar to that of FIG. 2 and showing the state in which the center of the ball 3 is at the position B of FIG. 4. As apparent from FIG. 5, the flank (arc) portion of the thread groove 2a which is in the upper and right side of the figure does not exist. The flank which exists in the normal state is shown by a phantom line. In other words, during a period when the center of the ball moves from the position C to the position A, the ball is contacted with the thread grooves 1a and 2a at three points.
In such a case, the reaction force Fn1 exerted between the ball 3 and the thread groove 2a of the nut 2 at the point N1 opposes to the reaction force Fs2 exerted between the ball 3 and the thread groove 1a of the screw shaft 1 at the point S2 to balance therewith. Because of the above-mentioned non-existence of the flank, however, no reaction force which opposes to the reaction force Fs1 exerted between the ball 3 and the thread groove 1a of the screw shaft 1 at the point S1 is produced. As a result, the ball 3 receives a force of FC (the force obtained by subtracting the friction force between the ball and the thread groove from the reaction force Fsl) in the direction of Fsl at the point S1.
In order to eliminate backlash a pre-load is applied between the balls and the thread grooves, so that the force FC pushes the ball 3 in the direction of the force, thereby pushing the ball 3 to bite the grooves in the direction in which a flank does not exist. Even in the case where a pre-load is not applied, when a load is externally applied, a force similar to the force FC is produced so as to cause the ball 3 to bite the grooves.
During the period when the center of the ball 3 moves from the position C to the position A in FIG. 4, therefore, a state in which the ball 3 is easily bitten by the thread grooves 1a and 2a arises and a problem in that maloperation such as torque variations, jerk or jamming easily occurs in the operation of the ball screw mechanism.